The present invention relates generally to test sensors and more particularly to electrochemical test sensors.
There are many medical conditions which require frequent measurement of the concentration of a particular analyte in the blood of a patient. For example, diabetes is a disease which typically requires a patient to routinely measure the concentration of glucose in his/her blood. Based upon the results of each blood glucose measurement, the patient may then require a particular drug treatment (e.g., an injection of insulin) in order to regulate that the blood glucose level of the patient remains within a specified range. Exceeding the upper limit of said range (hyperglycemia) or dropping beneath the lower limit of said range (hypoglycemia) should be avoided with as much diligence as possible to prevent the patient from experiencing serious medical complications which include, inter alia, retinopathy, nephropathy, and neuropathy.
A multi-step process is commonly practiced by diabetes patients to self-monitor the level of glucose present in their blood.
In the first step of said process, a patient is required to provide a blood sample suitable for testing. Blood samples taken from a patient for blood sugar monitoring are typically obtained by piercing the skin of the patient using a lancing device. A lancing device typically includes a body and a lancet. The body is typically adapted to be held by the user, the lancet being coupled to the body and being adapted to penetrate through the epidermis (the outermost layer of the skin) of the patient and into the dermis (the layer of skin directly beneath the epidermis) which is replete with capillary beds. The puncture of one or more capillaries by the lancet generates a sample of blood which exits through the incision in the patient's skin.
In some lancing devices, the lancet extends from the body at all times. In other lancing devices, the lancet is adapted to be moved, when actuated, from a retracted position in which the lancet tip is disposed within the body to an extended position in which the lancet tip extends beyond the body. Typically, the movement of the lancet from its retracted position to its extended position is effected with such force that contact of the moving lancet tip with the skin of a patient results in the piercing of the skin of the patient. In many such lancing devices having a movable lancet, the lancet is automatically drawn back into the body after reaching its extended position (e.g., using a spring) in order to minimize the risk of inadvertent lancet sticks.
In the second step of said process, a blood glucose monitoring system is utilized to measure the concentration of glucose in the blood sample. One type of glucose monitoring system which is well known and widely used in the art includes a blood glucose meter (also commonly referred to a blood glucose monitor) and a plurality of individual, disposable, electrochemical test sensors which can be removably loaded into the meter. Examples of blood glucose monitoring systems of this type are manufactured and sold by Abbott Laboratories of Abbott Park, Ill. under the PRECISION line of blood glucose monitoring systems.
Each individual electrochemical test sensor (also commonly referred to as an electrochemical test strip) typically includes a substrate which is formed as a thin, rectangular strip of non-conductive material, such as plastic. A plurality of carbon-layer electrodes are deposited (e.g., screen printed) on the substrate along a portion of its length in a spaced apart relationship, one electrode serving as the reference electrode for the test sensor and another electrode serving as the working electrode for the test sensor. All of the conductive electrodes terminate at their first ends to form a reaction area for the test sensor. In the reaction area, an enzyme is applied onto the first end of the working electrode. When exposed to the enzyme, glucose present in a blood sample undergoes a chemical reaction which produces a measurable electrical response (i.e., a current). The second ends of the electrodes are disposed to electrically contact associated conductors located in the blood glucose monitor, as will be described further below.
A blood glucose monitor is typically modular and portable in construction to facilitate its frequent handling by the patient. A blood glucose monitor often comprises a multi-function test port which is adapted to receive the test sensor in such a manner so that an electrical communication path is established between the second ends of the test strip electrodes and the electronic circuitry for the blood glucose monitor. Within the housing of the monitor, the test port is electrically connected to a microprocessor which controls the basic operations of the monitor. The microprocessor, in turn, is electrically connected to a memory device which is capable of storing a multiplicity of blood glucose test results.
In use, the blood glucose monitoring system of the type described above can be used in the following manner to measure the glucose level of a blood sample and, in turn, store the result of said measurement into memory as test data. Specifically, a disposable electrochemical test sensor is unwrapped from its packaging and is inserted into the test port of the monitor. With the test sensor properly inserted into the monitor, there is established a direct electrical contact between the second ends of the electrodes of the test sensor and the conductors contained within the test port, thereby establishing an electrical communication path between the test sensor and the monitor. Having properly disposed the test sensor into the test port, the monitor applies a voltage (e.g., 200 mv) across the second ends of the electrodes and automatically provides a “ready” indication on its display.
The user is then required to provide a blood sample using a lancing device. Specifically, a disposable lancet is unwrapped from its protective packaging and is loaded into a corresponding lancing device. The lancing device is then fired into the skin of the patient to provide a blood sample.
After lancing the skin, the patient is required to deposit one or more drops of blood from the patient's wound site onto the reaction area of the test sensor, the blood sample creating a conductive path between the first ends of the working and reference electrodes. When a sufficient quantity of blood is deposited on the reaction area of the test sensor, an electrochemical reaction occurs between glucose in the blood sample and the enzyme deposited on the first end of the working electrode which, in turn, produces an electrical current which decays exponentially over time. It should be noted that the value of this electrical current, which is commonly referred to in the art as the working current, is proportional to the concentration of glucose in the blood sample.
The decaying electrical current created through the chemical reaction between the enzyme and the glucose molecules in the blood sample travels along the working electrode and is measured by a current measuring device located within the monitor. The microprocessor of the monitor, which is connected to the current measuring device, correlates the measured working current into a standard numerical glucose value (e.g., using a scaling factor). The numerical glucose value calculated by the monitor is then shown on the monitor display for the patient to observe. In addition, the data associated with the particular blood glucose measurement is stored into memory.
Electrochemical test strips of the type described above are conventionally manufactured in batches. Due to inevitable inconsistencies in manufacturing, variances often arise between different batches of test strips (e.g., the size of the working and reference electrodes, the amount of enzyme deposited on the working electrode, etc.). These manufacturing variances have been found to directly alter the value of the working current produced when a blood sample is deposited on the test strip. As can be appreciated, any alteration of the value of the measured working current can render the glucose level reading calculated therefrom potentially inaccurate, which is highly undesirable.
Accordingly, there presently exist different methods for adjusting the value of the measured working current to compensate for such variances in manufacturing.
For example, in one well known adjustment method, an independent calibration strip is utilized to provide information relating to a batch of test strips to the blood glucose meter. Specifically, a batch of test strips is manufactured and then, in a subsequent step, a limited sampling of the test strips is tested for accuracy by the manufacturer using a blood sample of a known glucose level. Any deviation in the value of the working current generated from a test strip during this test is used to adjust the results obtained from future tests using the remaining test strips from the same batch. This adjustment is accomplished using an independent calibration strip which contains information relating to the deviation associated with the batch.
In use, a blood glucose meter is provided with a default calibration value. Prior to performing an assay, the separate calibration strip is inserted into the test port of the blood glucose meter. Information provided on the calibration strip is digitally transferred to the microprocessor which, in turn, adjusts the default calibration value for the meter. The adjusted calibration value is then used by the meter to correct future glucose readings which are taken using test strips from the same batch. For example, the calibration strip is often provided with a code which the blood glucose meter then converts to a particular numerical value. This numerical value is then utilized by the microprocessor to convert the working current from its measured (i.e., inaccurate) value to a compensated (i.e., true) value. The compensated value of the working current is then utilized by the meter to calculate an accurate blood glucose concentration reading. It should be noted that the particular code associated with a batch of test strips is often stored in memory on the calibration strip using at least one of the following means: a resistor, read only memory (ROM), a key code or a barcode.
The above-described use of an independent calibration strip to digitally calibrate a blood glucose meter prior to testing introduces some notable drawbacks. First, the use of an independent calibration strip requires a patient to perform the time-consuming and complicated task of digitally calibrating a blood glucose meter prior to performing an assay using a test strip from its associated batch. Second, the use of an independent calibration strip in conjunction with a batch of test strips significantly increases the overall manufacturing costs for said batch, this increase in manufacturing costs being a direct consequence of the costly memory requirement for the calibration strip.
Accordingly, in a second well-known adjustment method, calibration information is provided directly on each test strip in a particular batch (the calibration information, in turn, is used by the meter to digitally adjust the measured working current to its compensated, or actual, value). As an example, one well-known test strip utilizes a plurality of contact pads which can be interconnected in a particular pattern so as to represent a specific calibration code. Specifically, the test strip is manufactured with six isolated test pads which are interconnected through a series of conductive leads. After the batch of test strips has been manufactured, a sampling of the test strips is tested to determine whether manufacturing tolerances have introduced any inaccuracies. Based on the results of the testing, the manufacturer, in a subsequent step, cuts selected leads (e.g., using a laser) on each remaining test strip in the batch so as to interconnect the conductive pads into a particular pattern.
As such, when the user wishes to perform an assay, a reconfigured test strip is inserted into a compatible blood glucose meter. The meter reads the pattern of interconnected pads on the test strip and, in turn, corresponds said pattern into a particular calibration code. The meter then uses the calibration code to digitally convert the working current received during an assay to a compensated (i.e., true) value which can then be used to accurately calculate the blood glucose concentration of the sample.
One drawback associated with the aforementioned test strip is the limited number of calibration codes that it can accommodate. Specifically, the limited number of pads (as well as the limited number of leads) allows for the creation of a minimal number of patterns (typically no more than 10-15 patterns). As a result, only a small number of different calibration codes can be provided for such a test strip. However, it has been found that manufacturing tolerances often require a relatively large number of different calibration codes (e.g., often as many as 50 calibration codes are required). Consequently, the limited number of calibration codes afforded by such a test strip has been found to be inadequate.